readme: update installation instructions

* remove Python 2 (and dependants) from prerequisites
* mention ParMETIS not being available in Homebrew anymore (see #33)
* improve explanation of MAKE_FLAGS

README.md

@@ -78,11 +78,10 @@ Depending on your system configuration, there will be more packages necessary to
 | pkg-config |
 | HDF5 | | with MPI support
 | FFTW3 | | with MPI support
-| Python | 2.7 or 3.x |
-| pip | 2.7 or 3.x |
+| Python | 3.x |
+| pip | 3.x |
 | MPI | | Tested with OpenMPI
 | SuperLU | 4.3 or 5.x |
-| VirtualEnv | | For Python 2 only. Install via `pip` |
 | [dune-common](https://gitlab.dune-project.org/core/dune-common) | releases/2.5
 | [dune-geometry](https://gitlab.dune-project.org/core/dune-geometry) | releases/2.5
 | [dune-grid](https://gitlab.dune-project.org/core/dune-grid) | releases/2.5
@@ -104,7 +103,11 @@ Depending on your system configuration, there will be more packages necessary to
 
 
 ### Step-by-step Instructions
-These instructions are suitable for a clean **Ubuntu** or **Mac OS X** setup. The main difference between the two systems is the package manager. Debian-based systems have the APT manager already built in. On Mac, we recommend installing [Homebrew](http://brew.sh/). If you prefer to use [MacPorts](https://www.macports.org/), you need to check if some of the packages require different installation options than displayed here. 
+These instructions are suitable for a clean **Ubuntu** or **Mac OS X** setup. The main difference between the two systems is the package manager. Debian-based systems have the APT manager already built in. On Mac, we recommend installing [Homebrew](http://brew.sh/). If you prefer to use [MacPorts](https://www.macports.org/), you need to check if some of the packages require different installation options than displayed here.
+
+1. **Mac OS X** users need to start by installing the Apple Command Line Tools by executing
+
+        xcode-select --install
 
 1. Install third party packages:
     
@@ -130,9 +133,7 @@ These instructions are suitable for a clean **Ubuntu** or **Mac OS X** setup. Th
 
         apt install libmetis-dev libparmetis-dev
 
-    **Mac OS X:**
-
-        brew install metis parmetis
+    **Mac OS X:** _Support is dropped because ParMETIS is currently unavailable from Homebrew._
 
     **Parallel runs without these two packages are possible but not supported!**
 
@@ -144,11 +145,13 @@ These instructions are suitable for a clean **Ubuntu** or **Mac OS X** setup. Th
 
     to enfore the Homebrew'd GCC compiler. Note that this variable export only lasts for your current terminal session. Always make sure that the configuration tool actually finds GCC instead of the Apple Clang.
 
-4. Clone the DUNE modules and DORiE into a suitable folder on your machine. Make sure that you check out the correct branches (see above). Enter the parent folder, and call
+4. Clone the DUNE modules and DORiE into a suitable folder on your machine. Use `git checkout` to switch to the correct branches (see above). Enter the parent folder, and call
+
+        CMAKE_FLAGS="-DDUNE_PYTHON_VIRTUALENV_SETUP=True -DDUNE_PYTHON_ALLOW_GET_PIP=True" ./dune-common/bin/dunecontrol all
 
-        CMAKE_FLAGS="-DDUNE_PYTHON_VIRTUALENV_SETUP=True -DDUNE_PYTHON_ALLOW_GET_PIP=True" MAKE_FLAGS="-j <N>" ./dune-common/bin/dunecontrol all
+    to build all DUNE modules. Additionally, you can add `MAKE_FLAGS="-j X"` before the call to `dunecontrol` to compile on `X` processes in parallel.
 
-    to build all DUNE modules in parallel on `N` processes. If you installed software into paths not appended to your `PATH` variable, you will have to add `CMAKE_FLAGS` to the call to make sure that CMake finds all packages. Alternatively, you can add a custom options file. See the [DUNE Installation Instructions](https://dune-project.org/doc/installation/) for details. CMake will throw an error if required packages are not found.
+    If you installed software into paths not appended to your `PATH` variable, you will have to add `CMAKE_FLAGS` to the call to make sure that CMake finds all packages. Alternatively, you can add a custom options file. See the [DUNE Installation Instructions](https://dune-project.org/doc/installation/) for details. CMake will throw an error if required packages are not found.
  
 5. After a successful build you can call the `dorie` wrapper located in `dorie/build-cmake/bin` from anywhere on your machine, but it is more handy to add this location to your search path. You can add the directory to your `PATH` variable by calling